In a common rail fuel injection system, high pressure actuation fluid is used to power electronic unit injectors, and the actuation fluid is supplied to the injectors from a high pressure fluid accumulator, which is referred to as a rail. To permit variation of the fluid pressure supplied to unit injectors from the rail, it is desirable to vary the delivery of fluid to the rail from one or more actuation fluid pumps. Known common rail systems typically rely on either a single fluid pump that supplies fluid to the rail or a plurality of smaller displacement pumps that each supplies fluid to the rail. The volume and rate of fluid delivery to the rail has been varied in the past by providing a rail pressure control valve that spills a portion of the delivery from a fixed delivery pump to maintain the desired rail pressure.
Variable delivery pumps are well known in the art and are typically more efficient for common rail fuel systems than a fixed delivery actuation fluid pumps, since only the volume of fluid needed to attain the desired rail pressure must be pumped. For example, variable delivery has been achieved from an axial piston pump, e.g. a pump wherein one or more pistons are reciprocated by rotation of an angled swash plate, by varying the angle of the swash plate and thus varying the displacement of the pump. In such a pump, the swash plate is referred to as a “wobble plate”. Variable delivery has also been achieved in fixed displacement, axial piston pumps by a technique known as sleeve metering, in which each piston is provided with a vent port that is selectively closed by a sleeve during part of the piston stroke to vary the effective pumping portion of the piston stroke.
While known variable delivery pumps designs are suitable for many purposes, known designs are not always well suited for use with modern hydraulically actuated fuel systems, which require fluid delivery to the rail to be varied with high precision and with rapid response times measured in microseconds. In addition, known variable delivery pumps designs are typically complex, may be costly, and are subject to mechanical failure.
In one specific example, U.S. Pat. No. 5,630,609 to Kadlicko shows a fixed displacement swash plate type pump that achieves variable output via sleeve metering. The sleeve metering mechanism of Kadlicko appears to utilize a hydraulic force that is balanced against a spring force to adjust the position of the sleeve. In order to adjust the pump output, the positions of the metering sleeves are sensed and then fluid pressure is adjusted to move the sleeves to a different desired output position. The Kadlicko pump appears to suffer from several drawbacks, including its complex control strategy, which would appear to be accompanied by relatively difficult problems in calibrating control signals with desired outputs from the pump.
This invention is directed to overcoming one or more of the problems described above.